It is estimated that phosphate rock, which is a raw material for phosphorus, will be depleted in less than 100 years. Japan imports 100% of its phosphate rock, and phosphorus is indispensable in agricultural production. Thus, food shortages may become a serious problem if nothing is done. Against such a background, attempts have been made to recycle organic waste by recovering and recycling phosphorus from activated sludge used in sewage treatment, food wastewater treatment, livestock wastewater treatment, or the like. In particular, the phosphorus content in sludge has increased recently through advances in sewage treatment, and thus, reduction of environmental load by recovering phosphorus from sewage sludge has become important.
An example of a known sludge treatment method including phosphorus recovery involves: solubilizing sludge using ozone or ultrasonic waves; separating solids from liquids; and adding Ca or Mg ions to the separated liquid to precipitate the barely soluble phosphate, to thereby recover phosphorus (see JP 2003-047988 A, for example). Further, an example of a known sludge treatment method employing foaming involves: blowing an ozone-containing gas through sludge to form a gas-liquid contact region; and subjecting the sludge to ozone treatment while maintaining the gas-liquid contact region at a constant height (see JP 08-267099 A, for example). However, both the methods have problems in that phosphorus in the sludge can not be sufficiently eluted and the phosphorus recovery rate is low.
Thus, a method of eluting phosphorus in sludge more efficiently than conventional methods has been proposed, which involves: subjecting the sludge to ozone treatment; and then treating the sludge with an alkali such as sodium hydroxide (see JP 2003-200193 A, for example).
However, in conventional methods involving alkali treatment after ozone treatment, ozone is not efficiently used for reaction with the sludge even if a large volume of ozone-containing gas is blown because the ozone is wasted through a reaction with dissolved components. Thus, conventional methods have problems in that the phosphorus recovery rate is about 70%, which is far from being sufficient, and in that treatment cannot be carried out efficiently. Further, the methods have problems in that the residence time of the sludge had to be long for sufficient reaction to occur between the sludge and the ozone to reduce sludge volume because of the small gas-liquid contact area.